[Ada] Implement AI12-0175 Preelaborable packages with address clauses

[Ada] Implement AI12-0175 Preelaborable packages with address clauses

2020-06-12  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* rtsfind.ads (RTU_Id): Add System_Address_To_Access_Conversions.
	* sem_elab.adb (Elaboration_Phase_Active): Alphabetize.
	(Finalize_All_Data_Structures): Likewise.
	(Error_Preelaborated_Call): New procedure.
	(Build_Call_Marker): Set Is_Preelaborable_Call flag in marker.
	(Build_Access_Marker): Likewise.
	(Build_Subprogram_Invocation): Likewise.
	(Build_Task_Activation): Likewise.
	(Check_Preelaborated_Call): Return when the call is preelaborable.
	Call Error_Preelaborated_Call to give the error otherwise.
	(Check_Elab_Call): Likewise.
	* sem_util.adb (Is_Preelaborable_Function): New predicate.
	(Is_Non_Preelaborable_Construct.Visit): Recurse on the
	Explicit_Actual_Parameter field of N_Parameter_Association.
	(Is_Non_Preelaborable_Construct.Visit_Subexpression): In Ada 2020,
	for a call to a preelaborable function, visit the parameter list;
	otherwise, raise Non_Preelaborable exception.
	(Is_Preelaborable_Construct): Likewise, but recursively check the
	parameters instead and return false upon failure, otherwise true.
	* sinfo.ads (Is_Preelaborable_Call): New flag in call marker nodes.
	(Is_Preelaborable_Call): New inline function.
	(Set_Is_Preelaborable_Call): New inline procedure.
	* sinfo.adb (Is_Preelaborable_Call): New inline function.
	(Set_Is_Preelaborable_Call): New inline procedure.